An optical heterodyne interferometer combines signals output from a laser or other light source in measurement and reference channels. In a conventional displacement measuring heterodyne interferometer, position of a moving object, for example, is determined using differences in phase progression between a signal in the measurement channel and a signal in the reference channel. These phase progressions are end-products of two phase digitizers, as described for example in U.S. Pat. No. 6,480,126 to Chu (Nov. 12, 2002), which is hereby incorporated by reference.
As discussed, in metrology based homodyne interferometry, a phase progression function φ(t) is directly proportional to displacement of the moving object in time (t), usually by a factor of λ/4. Thus, one unit interval (UI) change, where one UI is 2π radians, represents object movement of one-quarter of the wavelength of the optical beam light wave. Metrology based on heterodyne interferometry involves two channels: one Doppler shifted (the measurement channel) and one not Doppler-shifted (the reference channel). The difference between a measurement channel phase progression function φM(t) and a reference channel phase progression function φR(t) is proportional to the displacement of the moving object to within an arbitrary constant. The phase progression functions φM(t), φR(t) are monotonically increasing with time.
More particularly, in the measurement channel, a Doppler shifted first optical beam, having optical frequency OF1, mixes with an un-modulated second optical beam, having optical frequency OF2. An electronic signal is generated, having frequency F2, which is the difference between the optical frequencies OF1 and OF2 (which may be referred to as “split frequency”). The frequency F2 is modulated by the velocity of the moving object. In the reference channel, an electronic reference signal is formed by mixing an un-Doppler shifted version of the first optical beam having optical frequency OF1 with the un-modulated second optical beam having optical frequency OF2. The reference signal is a steady electronic signal at the frequency F2. Note that the frequency F2, while not very agile, still changes over time and needs to be actively tracked.
The first and second optical beams may be separated expediently by polarization, although they are usually collinear and pass through common optical components. However, polarization separation is not perfect and one or both of the optical frequencies OF1 and OF2 may leak into the other. Leakage in polarization causes an interferometer signal to exhibit cyclic error in the phase digitized result. For example, the leakage causes the measurement channel to have electronic signals at three frequencies: a first signal having frequency F1, which is Doppler shifted about the frequency F2, a second signal having the frequency F2, which is typically a smaller perturbing signal, and a third signal having frequency F3, which is a typically an even smaller perturbing signal negatively Doppler shifted about the frequency F2. A conventional phase digitizer generally tracks the larger first signal having frequency F1 very well, and ignores the smaller perturbations resulting in cyclic error in the digitized result. It is difficult to effectively track all three frequencies F1 to F3 simultaneously in the measurement channel because they have different bandwidths and signal strengths. In addition, the three frequencies F1 to F3 may be very close (slow motion of the object) or even identical (no motion of the object) to one another.
Another method to eliminate cyclic error is to compensate for cyclic error in the digitized phase data in a separate step from phase digitizing process, for example, as described in U.S. Pat. No. 6,738,143 to Chu (May 18, 2004), which is hereby incorporated by reference. According to this method, the digitized phase progression is analyzed in batches of 320 phase-data to produce a measure of the cyclic error (referred to as non-linearity) present and the value is used to compensate the next 320 points individually. Latency of at least one millisecond is manifested. The measurement method also shows predictable inaccuracy depending on the velocity of the moving object. This must be accounted for if the compensation is to be accurate.
There is therefore a need for a phase digitizing system, e.g., for an optical heterodyne interferometer, that provides differences between signals in measurement and reference channels without cyclic error.